Automatic manual transmissions (commonly named “AMT”) are increasingly widespread, which transmissions are structurally similar to manual gearbox of the traditional type except for the clutch pedal and the gear selection lever operated by the driver being replaced by corresponding electric or hydraulic servo controls.
An automatic manual transmission is provided with a transmission control unit, which while shifting gears, drives the servo controls associated with clutch and gearbox to disengage the current gear and engage the successive gear; furthermore, while shifting gear, the transmission control unit interfaces with the engine control unit to conveniently adjust the torque delivered by the engine so as to adapt the engine operation to the transmission ratio change and to avoid an undesired temporary increase/decrease of engine rpm when the clutch is open or in all cases either while opening or closing. Such a control mode is applied both to traditional single clutch gearboxes and to double clutch gearboxes; the only difference between the two types of gearboxes resides in that in a single clutch gearbox there is a central interval of time in which the clutch is completely open (i.e. the engine idly turns without load), while in a double clutch gearbox the two clutches mutually cross over and therefore a clutch opening/closing situation exists during the whole shifting operation.
A clutch has an engine side angularly integral with the drive shaft of the engine (possibly by interposing a transmission shaft) and a gearbox side, integral with a primary shaft of the gearbox, which is in turn angularly integral with the driving wheels through the currently engaged gear; the rotation speed of the two sides of the clutch is adapted when closing the clutch, because when the clutch is completely closed, the two sides of the clutch are angularly integral with each other, and thus rotate at the same rotation speed. The rotation speed of the driving wheels is imposed by the forward traveling speed of the vehicle, and varies with relatively long times because of the high total inertia of the vehicle itself; instead, the rotation speed of the drive shaft of the engine may be quickly varied in a controlled manner when the drive shaft is idling (i.e. when it is separated from the driving wheels) due to the low total inertia of the engine. Therefore, in the known, currently marketed automatic manual transmissions, when closing a clutch, the rpm of the drive shaft (i.e. of the engine side of the clutch) is controlled according to the rpm of the gearbox side of the clutch (thus according to the rotation speed of the driving wheels, multiplied by the transmission ratio of the currently engaged gear), so as to make a connection between the rotation speed of the engine side of the clutch and the rotation speed of the gearbox side of the clutch as “smooth” as possible. Indeed, if the connection between the rotation speed of the engine side of the clutch and the rotation speed of the gearbox side of the clutch is “violent”, oscillations are triggered in the transmission, which cause longitudinal vibrations on the vehicle, and are thus perceived by the driver and by possible passengers.
However, in the known, currently marketed automatic manual transmissions, the clutch is closed very slowly in order to obtain a “smooth” connection between the rotation speed of the engine side of the clutch and the rotation speed of the gearbox side of the clutch; such a mode is effective in avoiding the onset of oscillations in the transmission, but on the other hand increases the time needed to shift gears, and thus penalizes performance. The problem is particularly evident in double clutch gearboxes, in which the clutches are in an oil bath because, while in a dry clutch used in a conventional single clutch gearbox the transmitted torque remains virtually constant despite slip variations (i.e. differences of rpm between the two sides of the clutch), in an oil bath clutch used in a double clutch gearbox, the torque transmitted by the clutch clearly drops to small slip values, and such a drop of the transmitted torque has a high dispersion from clutch to clutch. Therefore, in an oil bath clutch, controlling the torque transmitted by the clutch at small slip values (i.e. when connecting the rotation speed of the engine side of the clutch to the rotation speed of the gearbox side of the clutch is required) is very complicated.
US2008207393A1 describes a clutch control method which determines a torque target which is to be transmitted by the clutch during a gear shift to reduce the total time required for the gear shift itself. A clutch controller controls an opening degree of the clutch by driving an clutch actuator according to the difference between the torque actually transmitted by the clutch and a target of the torque transmitted by the clutch; moreover, the clutch controller determines whether a speed difference between the two sides of the clutch is reduced with an adequate acceleration and, according to such a determination, the clutch controller corrects the target of the torque transmitted by the clutch.